Problem: Simplify the following expression: $y = \dfrac{3x^2- 10x+3}{3x - 1}$
First use factoring by grouping to factor the expression in the numerator. This expression is in the form ${A}x^2 + {B}x + {C}$ First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(3)}{(3)} &=& 9 \\ {a} + {b} &=& &=& {-10} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $9$ and add them together. The factors that add up to ${-10}$ will be your ${a}$ and ${b}$ When ${a}$ is ${-1}$ and ${b}$ is ${-9}$ $ \begin{eqnarray} {ab} &=& ({-1})({-9}) &=& 9 \\ {a} + {b} &=& {-1} + {-9} &=& -10 \end{eqnarray} $ Next, rewrite the expression as $({A}x^2 + {a}x) + ({b}x + {C})$ $ ({3}x^2 {-1}x) + ({-9}x +{3}) $ Factor out the common factors: $ x(3x - 1) - 3(3x - 1)$ Now factor out $(3x - 1)$ $ (3x - 1)(x - 3)$ The original expression can therefore be written: $ \dfrac{(3x - 1)(x - 3)}{3x - 1}$ We are dividing by $3x - 1$ , so $3x - 1 \neq 0$ Therefore, $x \neq \frac{1}{3}$ This leaves us with $x - 3; x \neq \frac{1}{3}$.